Optoelectronic displays

ABSTRACT

Apparatus and methods for providing displays based upon the principle of photoluminescent quenching are described. The invention includes a method of displaying information using photoluminescence quenching, the method comprising providing an optoelectronic display comprising a photoluminescent material between a pair of electrodes, providing illumination for the photoluminescent material to cause the photoluminescent material to photoluminescence, and biasing the electrodes to at least partially quench the photoluminescence.

This is the U.S. national phase of International Application No.PCT/GB02/03935 filed Aug. 29, 2002, the entire disclosure of which isincorporated herein by reference.

FIELD OF THE INVENTION

This invention generally relates to displays and display drivers, andmore particularly to apparatus and methods for providing displays basedupon the principle of photoluminescence quenching.

BACKGROUND TO THE INVENTION

A broad range of optoelectronic display devices is known, ranging fromincandescent lamps and light emitting diodes through to liquid crystaland plasma displays and cathode ray tubes. A particularly importantcategory of displays is pixellated displays and, for low powerapplications, liquid crystal displays, (LCDs) are generally thetechnology of choice. Known LCD displays operate in transmissive mode,for example using a back light, reflective mode, for example illuminatedby daylight, and transflective mode in which the pixels have bothreflective and transmissive elements. However despite their popularityLCD displays still suffer from a number of problems which, even aftermany years of research, have not been fully overcome. Thus generallyspeaking LCD displays are relatively slow, with switching times of theorder of milliseconds, and have a relatively limited viewing angle. LCDsdisplays can also suffer from viewing artefacts such as displayinversion at certain viewing angles, and have a relatively unexcitingvisual appearance as compared with emissive display technologies.Furthermore since LCD displays are passive displays operating byblocking either transmitted or reflected light they have an upperefficiency limit which is reduced in colour displays by the need forcoloured filters.

Emissive display technologies overcome many of the above describedproblems and can provide a wide viewing angle and a bright, colourfuland interesting display with fewer visual artefacts. Known emissivedisplay technologies include cathode ray tubes, plasma display panels,thin film electro-luminescent displays, and organic light emittingdiodes (OLEDs), but a general problem with emissive displays is theirrelatively high power consumption which makes them unsuitable for manyapplications, and in particular, for many portable applications.

There therefore exists a general need to improve upon conventionaldisplays and to address the above problems, particularly the problems ofpower consumption and viewability.

Organic light emitting diodes (OLEDs) provide many advantages overbetter known display technologies including ease of fabrication andflexibility in the design of new materials for displays. Organic LEDshave been know for around ten years and can be based on eitherconjugated polymers or on smaller molecules although, generallyspeaking, the main features of devices based upon both these materialsare similar.

Typically an organic LED comprises a substrate on which a series oflayers is deposited including a pair of electrode layers to serve as ananode and cathode and, between these, a layer of electroluminescentorganic material. Optionally a hole-transporting layer is incorporatedbetween the anode and the electroluminescent layer and/or an electrontransport layer is incorporated between the electroluminescent layer andthe cathode. Generally heterostructures used for inorganic LEDs can bealso be adapted for organic LEDs.

In the case of polymer-based devices materials such as PPV(poly(p-phenylenevinylene)) may be used for the electroluminescent layerwhilst in the case of smaller molecule devices this layer may comprisematerials such as aluminium trisquinoline. The hole-transporting layermay comprise PEDOT (doped polyethylene dioxythiophene) in thepolymer-based devices or triarylamines in the smaller molecule devices.The electron-transporting layer may comprise oxadiazoles in the smallermolecule devices; there is generally no electron transporting layer inthe polymer devices. The anode typically has a higher work function thanthe cathode and is normally transparent to allow light to escape fromthe electroluminescent layer, often ITO (indium tin oxide) is used forthe anode.

Organic LEDs switch much faster than LCDs, typically in less than onemicrosecond. The flexibility of organic chemistry also makes itrelatively straight forward to synthesise new active materials fororganic LEDs as compared with inorganic LEDs, for example to allowtuning of the organic material's semiconductor band gap. A furtheradvantage of polymer LEDs is that they are relatively straight forwardto fabricate since deposition of the active layer can be performed atroom temperature using, for example, spin coating. Organic LEDs can alsobe formed on flexible substrates and patterned simply by pixellation ofone of the electrodes.

Further details of organic LED-based devices may be found in WO90/13148,WO98/59529, WO99/48160, WO95/06400, GB 2,312,326A, and U.S. Pat. No.5,965,901, all in the name of the present applicant, and all of whichare hereby incorporated by reference.

Notwithstanding the significant advantages which organic LEDs provideover more conventional display technologies, there is still a need forlower power consumption display devices with longer lifetimes.

The present invention stems from research work carried out into organiclight emitting diodes but is based upon an entirely new principle in thefield of optoelectronic displays. In particular, the applicant hasrecognised that the electroluminescent materials normally used inorganic light emitting diodes are usually also photoluminescent and thatthis photoluminescence may be reduced or quenched by applying anelectric field to the photoluminescent material when incorporated in asuitable structure. Suitable structures include conventional OLEDstructures and the electric field necessary to quench thephotoluminescence can be applied to the photo-(or electro-)luminescentmaterial simply by reverse biassing the OLED device, although thisphotoluminescence quenching effect is difficult to observe underordinary circumstances. The applicants have also recognised that theidea of using photoluminescence quenching to display information is not,in principle limited to the device structure and materials used fororganic LEDs but could also be applied to device structures andmaterials used for inorganic LEDs.

WO98/41065 discloses the application of either polarity of drivingvoltage to an electroluminescent polymer-based display to drive eitherred light emission from an interface of the polymer or green lightemission from the bulk of the polymer. However, in both cases, the lightemitting semiconductor is forward biassed (the device effectivelycomprises two back-to-back diodes). U.S. Pat. No. 6,201,520 describesthe use of reverse biassing for non selected pixels in a pixellated OLEDdisplay to prevent cross talk which could otherwise be caused by the(electrically) semi-excited state of the non-selected pixels. U.S. Pat.No. 5,965,901 describes the use of a pulse driving scheme for an organiclight-emitting polymer device to improve device lifetime in whichpositive pulses are separated by negative (reverse bias) pulses. U.Lemmer et. al., Synthetic Metals, 67 (1994) 169-172 describes theexperimental observation of photoluminescence quenching in an ITO/PPV/Alstructure. However none of these prior art documents discloses a displaybased upon the photoluminescence quenching principle or the use ofphotoluminescence quenching to provide a display.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is thereforeprovided a method of displaying information using photoluminescencequenching, the method comprising providing an optoelectronic displaycomprising a photoluminescent material between a pair of electrodes;providing illumination for the photoluminescent material to cause thephotoluminescent material to photoluminesce; and biassing the electrodesto at least partially quench said photoluminescence.

Displaying information using photoluminescence combines the durabilityadvantages of emissive display technologies with the low powerconsumption of reflective and transflective LCD displays. Since themethod relies on the control or modulation of photoluminescence it isbest categorised as an emissive technology and is capable of providingthe advantages associated with such technology, namely bright saturatedcolours and a Lambertian emission profile—that is an output intensitywhich is substantially constant over a range of angles, which assists inachieving a wide viewing angle. However because only a small current isneeded to quench the photoluminescence the power consumed by this methodof displaying information can be very low.

Although an illumination source is needed, this illumination may beprovided by ambient illumination such as daylight, and the method thusprovides the further advantage of good display viewability in brightconditions. Alternatively where a light source associated with theoptoelectronic display is used to provide the illumination, this lightsource can itself be selected for high efficiency. In either case formany materials the photoluminescence efficiency is far greater than theelectroluminescence efficiency, typically greater than 80% as comparedwith 1 to 5% for electroluminescence. The illumination for thephotoluminescence material may be visible, for example blue, orinvisible, for example ultraviolet, but should have a wavelength lessthan or equal to the photoluminescence wavelength. Thus greenillumination, for example, is adequate for a yellow or red display.

A further advantage, at least insofar as concerns organicphotoluminescence materials, is the potential for increased devicelifetime. In a conventional electroluminescent OLED displayelectro-migration plays a significant factor in the eventual degradationof the organic electroluminescer. The contrast in the present methodwhen organic photoluminescent material is biased so as to quench thephotoluminescence much less current flows and this decay mechanismbecomes less important. A further consequence is that many morematerials are available for use for displaying information usingphotoluminescence quenching than are available for conventionalelectroluminescent displays since materials which may have a relativelyshort lifetime under forward bias may still exhibit an acceptably longlifetime when biased to quench the photoluminescence.

The method may be used with any photoluminescent material, eitherorganic (large or small molecule), organometallic or inorganic, such asgallium arsenide or some other III-V or II-VI material However whenorganic materials are employed the above described benefits of OLEDtechnology, relating to material synthesis and device fabrication, mayalso be realised.

In preferred embodiments of the method the optoelectronic display is anactive matrix display such as a thin-film-transistor (TFT)-drivendisplay, or a DC driven display. Using a TFT-driven display simplifiesinterfacing and, because the photoluminescence quenching switches offvery quickly, facilitates achieving a good display contrast.

Preferably, the photoluminescent material comprises an organicphotoluminescent material such as a small molecule or a semiconductingconjugated organic polymer. Fluorine-based organic materials areparticularly preferable because of their high photoluminescentefficiency.

In a preferred embodiment the pair of electrodes comprises a cathode andan anode, the cathode having a lower work function than the anode. Theanode may be formed from ITO or a metal such as gold or silver; thecathode may be formed from a metal such as aluminium, calcium orlithium. Preferably the device is then reverse biased to make the anodemore negative than the cathode. When reverse biassed in this way thepower consumption may be little more than that required to conduct awayphoto-excited holes and electrons to quench the photoluminescence. In apreferred variant the optoelectronic display also includes a holetransport material between the photoluminescent material and the anodeto increase the efficiency of photoluminescence quenching.

Preferably at least one of the electrodes is at least partiallytransparent as it simplifies construction of the device and allowsprovision of a relatively large display surface area. Howeverside-emitting embodiments are also possible. Where at least oneelectrode is at least partially transparent, this electrode may also beused for illuminating the photoluminescent material, although thematerial may also be illuminated directly. An electrode need not coverthe entire surface of the photoluminescent material and one of theelectrodes may be confined to a spot or region of the device but thephotoluminescence quenching will only occur where an anode and cathodeoverlap.

In one embodiment of the method ambient or background light, such asdaylight or interior room lighting, may be used to provide theillumination. Alternatively illumination may be provided by a dedicatedlight source such as a back or preferably front-light. In still otherembodiments of the method both forms of illumination may be employedand/or a dedicated source of illumination may be employed according tothe level of ambient or background illumination. Where a back- orfront-light is employed it is preferable that both electrodes are atleast partially transparent, although this is not necessary where sideillumination is employed. Where artificial illumination is employed thismay be selected for its high efficiency and/or high output atwavelengths stimulating the photoluminescence. In some embodiments alight absorbing material may be incorporated into the optoelectronicdisplay to at least partially absorb the illumination and hence increasethe contrast between the photoluminescence and any background orscattered portion of said illumination.

Where the optoelectronic display extends over an area the illuminationmay be arranged to be generally perpendicular to the display area, forexample by shining light on the device from the side or from to one sideat the front or back of the device. However one particularly preferredembodiment of the method involves illuminating the photoluminescentmaterial by waveguiding light through the substrate on which the deviceis mounted. For example where the photoluminescent layer is sandwichedbetween a substantially transparent and a reflecting electrode theilluminating light may be waveguided between the reflecting electrodeand a front surface (towards an observer) of the substrate, the devicebeing mounted on the back surface of the substrate. The substrate may bemade of, for example, glass or plastic. The illuminating light may beconducted in from one or more sides of the device.

Using waveguided illumination has the advantage of being able tosubstantially confine the illumination within the waveguiding region sothat an observer sees the photoluminescence without any significantvisible background from the illumination. This helps to increase thecontrast in the display. The illumination preferably employs lighthaving a wavelength less than or equal to the photoluminescencewavelength, for example light with a wavelength shorter than 500 nm, orshorter than 450 nm, or shorter than 400 nm, or shorter than 350 nm.

It will be appreciated that where any form of dedicated light source orartificial illumination is used to illuminate the photoluminescentmaterial the relative contrast may be improved by using light of acolour or wavelength to which the human eye is relatively lesssensitive. Thus, for example, the sensitivity of the human eye fallsaway rapidly below 450 nm and therefore by using illumination with awavelength or peak wavelength in this region the effect of anybackground or scattered illumination on the photoluminescence displaymay be attenuated.

In one embodiment of the method a photoluminescent material is employedwhich becomes substantially colourless when the photoluminescence isquenched. Although, in practice, the photoluminescence may not beentirely quenched, by using a material which becomes or which wouldbecome substantially colourless when the photoluminescence is quenchedor not present the effect of any intrinsic colour of thephotoluminescent material on the display is reduced. Thus, for example,where the photoluminescent material is strongly coloured this may beapparent under ambient light or under illumination when thephotoluminescence is reduced or turned off. This can cause the apparentcolour of the display to change between an “on” photoluminescent stateand an “off” reduced-photoluminescence state, which may not be desirablefor some applications. Thus it is generally preferable to use aphotoluminescent material which, at least to the human eye, isintrinsically substantially colourless.

In one embodiment a photoluminescent material is used which comprises ablend of materials with different photoluminescent colours. This allowsthe display of “colours” which do not correspond to a pure singlewavelength emission. Thus in one embodiment a blend of materials whichphotoluminesces with a substantially white “colour” may be employed.This facilitates a black and white display which is advantageous for,for example, word processing.

In one embodiment the method uses a plurality of photoluminescencedisplay elements each having an associated pair of electrodes betweenwhich photoluminescent material is located. Thus, for example, an x-ymatrix of electrodes may be employed or a single common anode may beemployed with a separate cathode for each display element. Such anarrangement allows a pixellated display and where two or more differentphotoluminescent materials are employed which photoluminesce withdifferent colours a multi-coloured display can be provided. For example,a pixellated OLED-type display device structure may be illuminated andreverse biased to provide a colour photoluminescence quenching-typedisplay. Optionally, but preferably, each pixel has one or moreassociated transistors and/or one or more capacitors to allow the pixelto be maintained in a photoluminescence quenching state whilst anotherpixel is addressed.

In another embodiment of the method where the photoluminescent materialis also electroluminescent the optoelectronic display may be operated ina dual mode by, in effect, forward biassing the photo/electroluminescentmaterial when the display is in an “on state” to enhance the lightemitted from the display. In this case, however, it is preferable toemploy photoluminescent materials which have usefully longelectroluminescent lifetimes.

In one embodiment the method further comprises providing theoptoelectronic display with an optical structure to collect and deliverlight to the photoluminescent material. This is particularlyadvantageous where the illumination comprises ambient lightillumination. The optical structure preferably comprises a microstructure such as a micro-lens array. In such an optical micro structurethe lens or feature size is generally less than 1 mm, and often lessthan 0.1 mm, less than 10 micrometers or even less than 1 micrometer.

In a second aspect the invention provides a use of an optoelectronicdisplay comprising a photoluminescent material between a pair ofelectrodes to display information, the use comprising illuminating thephotoluminescent material to stimulate photoluminescence and; applying avoltage to the electrodes to quench the photoluminescence to displayinformation.

Preferably the display comprises a diode and the applied voltage reversebiases the diode. Ambient light alone, without additional dedicatedillumination, may be employed to illuminate the photoluminescentmaterial or a dedicated illumination source may be used to stimulate thephotoluminescence, or one or other of these illumination methods may beselected according to, for example, the ambient light level.

In a related aspect the invention provides a use of a display driver todrive a light emitting display to display information, the lightemitting display comprising a photoluminescent material between a pairof electrodes, the display driver applying a voltage of a first polarityto the electrodes to reduce photoluminescence from the material toswitch the light emission off and applying a reduced voltage at thefirst polarity, or substantially zero voltage, to the electrodes toswitch the light emission on.

The voltage may be applied to reduce or substantially quench thephotoluminescence. It will be appreciated that, depending upon theapplication, the light emission may only be switched “off” by comparisonwith a strongly emitting “on” state, and that to switch the displaylight emission off does not necessarily imply reducing photoluminescentemission to zero.

The use may further comprise modulating or changing a pulse width orduty cycle of the applied voltage to control the photoluminescence. Thusthe applied reverse bias voltage may be pulse-width modulated to providethe effect of an adjustable level of photoluminescence quenching. Insuch an arrangement, the applied voltage is switched between a firstlevel and a second level to provide a pulse train with an adjustablemark:space ratio. The first voltage level may correspond tosubstantially zero applied voltage and the second voltage level toreverse bias, for example to reduce or substantially quench thephotoluminescence.

The voltage is switched between these two levels at such a rate that, tothe human eye, the switching is not apparent and instead the effect isto vary the apparent degree of photoluminescence quenching dependentupon the mark:space ratio of the pulse train. Switching frequencies of25 Hz or greater, preferably, 60 or 100 Hz or greater are sufficient.Where a “space” corresponds to substantially quenched photoluminescenceand a “mark” to a substantially full-on display, a mark:space ratio of50% provides a half-on display and the display may be varied betweenfully on and fully off by varying the mark:space ratio between 100% and0%. In this way, a grey-level display may be provided.

In another aspect the invention provides a method of operating aphotoluminescent device, the device comprising a semiconductor layer inthe form of a film of organic photoluminescent material, a firstelectrical contact layer proximate a first surface of the semiconductorlayer, and a second electrical contact layer proximate a second surfaceof the semiconductor layer, the method comprising illuminating thedevice and applying an electric field between the first and secondcontact layers across the semiconductor layer so as to render the secondcontact layer negative relative to the first contact layer to splitoptically excited excitons generated by the illumination into theirconstituent holes and electrons and to conduct said holes and electronsout of said photoluminescent film to inhibit photoluminescence from saidfilm.

By conducting the optically excited excitons out of thephotoluminescence film the recombination of the holes and the electronsis inhibited, and thus the photoluminescence is attenuated. Where theorganic photoluminescent material comprises a conjugated polymer thesemi-conductor band gap is typically in the range of 1 eV to 3.5 eV.

As described above the method may further comprise illuminating thedevice by the guiding light between the first and second contact layers.Preferably the device is illuminated from the side, that isapproximately perpendicular to the display surface, and particularlypreferably the light is guided within the substrate, which istransmissive at the illumination and photoluminescence wavelengths. Theillumination must be of a shorter wavelength than the desiredphotoluminescence and it is therefore preferable that the illuminationis towards the blue end of the spectrum to facilitate the production ofa range of photoluminescence colours. Ultraviolet illumination may beemployed, with the advantage that such illumination will not be visible,although ultraviolet illumination sources have drawbacks related tocost, efficiency and safety. Generally the illumination source may beselected according to cost and power consumption and the desiredwavelength of photoluminescent emission.

Preferably the first contact layer has a lower work function than thesecond contact layer so that, relatively speaking, the first contactlayer is the better electron injecting material and the second contactlayer the better hole injecting material (under forward bias). Thisfacilitates removal of the charge carriers from the photoluminescentlayer when the device is reverse biased to reduce or quench thephotoluminescence.

In a preferred embodiment the film of organic material comprises a thin,dense polymer film, that is, the polymer film is not fibrillar and issubstantially free of voids. Preferably the film is also relatively freeof defects which act as non-radiative recombination centres as suchdefects will tend to reduce the overall photoluminescent efficiency. Oneor both of the contact layers may include an additional hole or electrontransport layer, preferably of an organic material. The polymer maycomprise a single conjugated polymer or a single co-polymer containingsegments of conjugated polymer or a blend of conjugated polymer orco-polymer with another suitable polymer.

Other generally preferred features of the organic material are physicaland chemical stability and processability.

Since the purpose of the first and second contact layers is to apply anelectric field across the device, it will be recognised that there isnot necessarily a requirement for a direct electrical connection betweenthese layers and the film of organic photoluminescence material itself.Providing that holes and electrons resulting from optically excitedexcitons can be inhibited from recombining this is sufficient to providephotoluminescence reduction or quenching. Thus, for example, theseparated holes and electrons could leak away or be drained away toprevent radiative recombination. A direct electrical connection ispreferable however to reduce the driving voltage required.

In another aspect the invention provides an optoelectronic displaydevice comprising: a semiconductor layer in the form of a film oforganic photoluminescent material, a first electrical contact layerproximate a first surface of the semiconductor layer, and a secondelectrical contact layer proximate a second surface of the semiconductorlayer; and a light source to illuminate said photoluminescent materialto stimulate photoluminescence from the material.

The invention also provides an optoelectronic display comprising aphotoluminescent display device having a display on-state in which thedisplay emits photoluminescence under optical illumination with novoltage applied to the device and a display off-state in which saidphotoluminescence is at least partially quenched; and device drivercircuitry having an input for receiving a display signal and an outputfor driving said display device, said display signal having an on-stateindicating that the display is to be on and an off-state indicating thatthe display is to be off; said photoluminescent display devicecomprising a semiconductor layer in the form of a film of organicphotoluminescent material, a first electrical contact layer proximate afirst surface of the semiconductor layer, and a second electricalcontact layer proximate a second surface of the semiconductor layer; andwherein said device driver circuitry is configured to apply an electricfield between the first and second contact layers across thesemiconductor layer so as to render the second contact layer negativerelative to the first contact layer to split optically excited excitionsgenerated by the illumination into their constituent holes and electronsand to conduct said holes and electrons out of said photoluminescentfilm to inhibit photoluminescence from said film in response to saiddisplay signal having said off-state; said display device and devicedriver combination primarily operating to display information byphotoluminescence quenching.

In one embodiment the device driver circuitry is further configured toreduce, but not to reverse, the electric field in response to thedisplay signal having its “on” state. The photoluminescence quenchingneed not be complete since in some circumstances a reduced contrastdisplay with only partial photoluminescence quenching may be acceptable.The degree of photoluminescence quenching may be varied either byvarying the electric field, that is, by providing a variable amount ofnegative bias on the display device or by varying the waveform of theapplied voltage. The device driver circuitry may either provide asingle-ended or differential output for driving the photoluminescencedisplay.

The device driver circuitry may also incorporate means to drive one ormore display pixels with a pulse-width modulated signal to provide anadjustable level of photoluminescence quenching. Thus the device drivercircuitry may incorporate means to receive an input signal specifying adesired level of photoluminescence and means responsive to the inputsignal to drive a pixel of the display with a pulse train with amark-space ratio dependent upon the input signal.

In a still further aspect the invention provides an optoelectronicdisplay comprising a semiconductor layer in the form of a film oforganic photoluminescent material, a first electrical contact layerproximate a first surface of the semiconductor layer, and a secondelectrical contact layer proximate a second surface of the semiconductorlayer; and a region to channel illumination from a light source usinginternal reflection, to illuminate said photoluminescent material.

Preferably the illumination is channelled using total internalreflection, using either step or graded index waveguiding. The displaymay also incorporate means such as a cylindrical lens to couple lightinto the waveguiding region.

In another aspect the invention provides an optoelectronic displaycomprising a semiconductor layer in the form of a film of organicphotoluminescent material, a first electrical contact layer proximate afirst surface of the semiconductor layer and a second electrical contactlayer proximate a second surface of the semiconductor layer, a substratecarrying said semiconductor layer and said first and second contactlayers; and an optical structure on said substrate to collect anddeliver light from said photoluminescent material to a viewer of thedisplay.

Preferably the optical structure comprises a plurality of microlenses.This assists in collecting ambient light and in directingphotoluminescent emitted light towards an observer of the display.

In another aspect the invention provides a pixellated optoelectronicdisplay comprising a plurality of photoluminescent display devices eachassociated with a pixel of the display and having a pair of electrodesfor addressing the device and device driver circuitry for driving theelectrodes to control the display, the pixels of the display having anormally-on photoluminescence emissive state under zero bias across theelectrodes, the display driver circuitry being configured to apply biasvoltage to inhibit said photoluminescent emission from selected pixelsof the display, to thereby display information.

The invention also provides optoelectronic device driver circuitry asdescribed above.

In a further aspect the invention provides an optoelectronic display asoperating on the principle of quenched photoluminescence, the displaycomprising a first electrode, a second electrode; and a visible displayelement located between the first and second electrodes, the displayelement comprising photoluminescent material, the device beingconfigured to at least partially quench photoluminescence from saidphotoluminescent material upon application of a voltage between saidfirst and second electrodes and thereby visibly change from aphotoluminescent emissive state to a reduced emissivity state to providea visual display.

The invention further provides a combination of an optoelectronicdisplay device and instructions for use of the device, the displaydevice comprising a semiconductor layer in the form of a film of organicphotoluminescent material, a first electrical contact layer proximate afirst surface of the semiconductor layer, and a second electricalcontact layer proximate a second surface of the semiconductor layer theinstructions comprising instructions to apply an electric field betweenthe first and second contact layers across the semiconductor layer so asto render the second contact layer negative relative to the firstcontact layer to inhibit photoluminescence from said photoluminescentfilm.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further described,by way of example only with reference to the accompanying figures inwhich:

FIGS. 1 a and 1 b show photoluminescence quenching device structures;

FIGS. 2 a and 2 b show spectra of photoluminescent materialsillustrating photoluminescence quenching;

FIGS. 3 a and 3 b show, respectively, schematic illustrations ofphotoluminescence from reflective and transparent cathode devices;

FIGS. 4 a and 4 b show illumination arrangements for photoluminescencequenching devices and FIG. 4 c shows an optical micro structure for aphotoluminescence quenching device;

FIGS. 5 a to 5 c show, respectively, cross-section and plan views of alighting arrangement for waveguiding illumination of a photoluminescencequenching device, and details of waveguiding within a photoluminescencelayer;

FIGS. 6 a to 6 d show, respectively, a pixellated photoluminescencequenching display, a pixellated colour photoluminescence quenchingdisplay and display driver, a pulse-width modulated waveform for drivinga photoluminescence quenching display, and an active matrix pixel drivercircuit;

FIG. 7 shows experimental apparatus for characterising photoluminescencequenching;

FIGS. 8 a and 8 b show photoluminescence quenching signals for twodevices measured using the apparatus of FIG. 7;

FIG. 9 shows photoluminescent intensity as a function of illuminationwavelength for the device of FIG. 8 a; and

FIG. 10 shows a possible theoretical mechanism for photoluminescencequenching.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring first to FIG. 1 a, this shows a cross section through thestructure of a basic device 100 suitable for use in a photoluminescencequenching display. The structure of the device is the same as that of aknown organic light emitting diode as described in the prior artdocuments mentioned in the introduction to the application. This isbecause, generally speaking, most organic LEDs will function as aphotoluminescence quenching display when reverse biased. However inpractice it may preferable to choose materials for use in aphotoluminescence quenching display according to a slightly differentset of parameters than that generally used for OLEDs, as explained inmore detail below. The fabrication techniques normally applied inconstructing OLEDs are also suitable for constructing photoluminescencequenching displays.

In the basic device of FIG. 1 a a glass substrate 102 supports an anodelayer 104, typically comprising indium tin oxide (ITO), which has goodtransparency, low sheet resistance and established processing routes. Alayer of photoluminescent material 106 is deposited on anode layer 104,the photoluminescent material comprising a conjugated organic polymer,or polymer blend, or a conjugated polymer doped with a photoluminescentmaterial. Since virtually all electroluminescent materials are alsophotoluminescent conventional electroluminescent materials using organicLEDs may also be employed in layer 106. Thus, for example, layer 106 maycomprise PPV (poly(p-phenylenevinylene)). A cathode layer 108 isdeposited over photoluminescent material layer 106 and a contact wire112 is bonded to the cathode, for example using silver dag 110(electrically conducting paint) containing colloidal silver. A similarconnection (not shown) is made by contact wire 116 to anode layer 104.Anode layer 104 has a relatively high work function, for example between4 eV and 5.2 eV, whilst cathode 108 has a relatively low work function,for example less than 3.5 eV. A power supply, illustratively shown bybattery 114, reverse biases this conventional OLED structure, applying apositive voltage to cathode 108 and a negative voltage to anode 104.

The photoluminescent layer 106 is illuminated by light 118 passingthrough the transparent substrate 102 and the transparent anode 104.Either a dedicated light source or background light or daylight may beemployed for illumination. Under quiescent conditions, with no appliedvoltage, light 118 causes photoluminescent layer 106 to luminesce andthis photoluminescence is visible through substrate 102 and anode 104.Thus in the illustrated embodiment substrate 102 forms the front face ofthe display and the quiescent condition of the display is “on” orlight-emitting. In practice the appearance of the display may dependboth upon the colour and intensity of photoluminescence and upon theintrinsic colour of photoluminescent layer 106, that is the colour thislayer would appear were the photoluminescence not present. Thecontribution of the intrinsic colour of photoluminescent layer 106 tothe display colour depends, in part upon how much of incidentillumination 118 is scattered back towards an observer of the display.

When, as shown in FIG. 1 a, the conventional OLED structure is reversebiased the photoluminescence from layer 106 is at least partiallyquenched resulting in a dimming of the display and where the quenchingis complete, the display is switched off. However the display is notnecessarily colourless or black when switched off as some residual,intrinsic colour from photoluminescent layer 106 for some residualreflectance from cathode 108 or other layers of the device may bepresent.

FIG. 1 a shows a cross-section through a simple photoluminescencequenching device but, in practice, a more complex structure asillustrated by the cross-section of FIG. 1 b is often preferable. InFIG. 1 b an additional hole transport layer 128 is present between anodelayer 104 and photoluminescent layer 106. This hole transport layerhelps match the hole energy levels of the photoluminescent layer withthose of the anode layer 104. A number of such hole transport layers maybe provided and, in a similar way, one or more electron transport layersmay be provided between the cathode and the photoluminescent layer. InFIG. 1 b cathode 122 comprises two layers, a first layer 124 having alow work function, for example a metal such as magnesium or aluminium,and a second layer 126 having a still lower work function, for example ametal such as calcium, lithium or barium or a metal fluoride.Combinations of metals, such as for example a LiAl combination, may alsobe used. This helps to match the electron energy levels in the cathodeand photoluminescent layer.

Any or all of hole and electron transporting layers and multilayercathodes and anodes may be employed.

The anode preferably has a work function of greater than 4.3 eV, and maycomprise indium oxide or indium tin oxide or a thin, partiallytransmissive high work function metallic anode such as a thin film ofgold or silver. Other materials such as fluorine-doped tin oxide andaluminium-doped zinc oxide can also be used, although it is preferablethat the sheet resistant of the anode is low, preferably less than 100Ohms/square, more preferably less than 30 Ohms/square. Metallic layersof 20 nm thickness, and more generally less than 50-100 nm have beenfound to be sufficiently optically transparent. Other metals such asaluminium, may, however, also be employed and in some embodiments, forexample where the cathode rather than the anode is at least partiallytransparent, the anode need not be transparent.

The cathode preferably has a work function of less than 3.5 eV and maycomprise, for example, barium, calcium, lithium, samariom, ytterbium,terbium, aluminium or an alloy comprising one or more of these metalswith or without another metal. Like the anode the cathode may be made atleast partially optically transmissive by depositing only a thin film ofmetal.

Although metals and metal-based compounds are convenient for use in theanode and cathode, other materials such as conducting polymers and dopedsemi-conductors may also be employed. Preferably electrode materialsshould have a registivity of less than 10,000 Ohm cm, more preferablyless than 1,000 Ohm cm. The anode and cathode materials are preferablyalso selected so that electrons and holes are not injected into thephotoluminescent layer 106 when a reverse bias is applied to the deviceas this could cause electroluminescent emission and breakdown.

The hole transport layer 128 may comprise polystyrene-sulphonate-dopedpolyethylene dioxythiophene(PEDOT:PSS—poly(ethylenedioxythiophene):poly(styrenesulphonic acid)) asfor example described in UK patent application number 9703172.8. Howeverother materials may also be used and in particular other polymers suchas poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-(4-imino(benzoicacid))-1,4-phenylene-(4-imino(benzoic acid))-1,4-phenylene)) (“BFA”)and/or polyaniline (doped, undoped or partially doped) and/or PPV.

The photoluminescent layer 106 may comprise a photoluminescentconjugated organic polymer or polymer blend or a conjugated polymerdoped with a photoluminescent material. Alternatively a so-called smallmolecule such as tris-(8-hydroxyquinolino aluminium)(“Alq3”) asdescribed in U.S. Pat. No. 4,539,507, may be employed. Suitable polymermaterials include PPV,poly(2-methoxy-5-(2′-ethyl)hexyloxyphenylenevinylene)(“MEH-PPV”), a PPVderivative (e.g. a di-alkoxy or di-alkyl derivative), a polyfluoreneand/or a co-polymer incorporating polyfluorene segments, PPVs and/orrelated co-polymers, poly(2,7-(9,9-di-n-octyfluorene)-(1,4-phenylene-((4-secbutylphenyl)imino)-1,4-phenylene))(“PFB”),(“TFB”)poly(2,7-(9,9-di-n-octylfluorene)-(1,4-phenylene-((4-methylphenyl)imino)1,4-phenylene-((4methylphenyl)imino)-1,4-phenylene))(“PFM”),poly(2,7-(9,9-di-n-octyfluorene)-(1,4-phenylene-((4-methoxyphenyl)imino)-1,4-phenylene-((4-methoxyphenyl)imino-14-phenylene))(“PFMO”),poly (2,7-(9,9-di-n-octyfluorene) (“F8”) or poly(2,7-(9,9-di-n-octylfluorene)-3,6-Benzothiadiazole) (“F8BT”). Furthersuitable materials and parameters for their selection are describedbelow with reference to FIG. 2.

The substrate 102 provides mechanical support and electrical insulationfor anode layer 104 and, in the embodiments of FIGS. 1 a and b, istransparent to allow photoluminescent layer 106 to be viewed. Suitablesubstrate materials include glass and clear plastics such aspolyethylene or PET. Other suitable substrate materials includepolyvinylidene fluoride and polyimide.

The illuminating light 118 may be red, green, blue, ultraviolet, orsubstantially white but must include sufficient wavelength components toexcite photoluminescent emission of the required colour. Generallyphotoluminescent emission is at a longer wavelength than the excitingillumination, and different display colours may therefore requiredifferent wavelengths of illuminating light—illumination with a blue orshorter wavelength component is required for blue photoluminescencewhilst, for example green illumination may be sufficient for redphotoluminescence.

Conventional organic LED fabrication techniques may be employed toconstruct the devices of FIGS. 1 a and b. Thus the anode and cathodeelectrode layers may be deposited by methods such as evaporation and RFand DC sputtering whilst the organic photoluminescence layer 106 andoptional hole transport layer 128 may be deposited by spin coating orfor larger areas, draw-coating or other techniques such as dip-coating,blade coating, miniscus-coating and self-assembly. The resulting pixelsare around 100 nm thick. So called “small molecules” may also bedeposited by sublimation.

The organic layers may also be deposited by ink-jet printing which hasthe advantage of facilitating patterning of the photoluminescent layer.Conventional ink-jet printing process polymers may be employed to givethe necessary control of surface tension and viscosity. A suitableink-jet printer spraying cycle is 14,400 drops per second, with a dropvolume of 30 pl. Ink jet printing is especially suitable formulti-colour displays. Where a conductive polymer is used as one of theelectrodes this may also be deposited by an ink-jet printing process.

Referring now to FIG. 2, this shows exemplary spectra of two differenttypes of photoluminescent material in FIGS. 2 a and 2 b. The spectrum ofFIG. 2 a is typical of materials which although having a relatively highphotoluminescence efficiency also have a strong intrinsic colour. Anexample of such a material is the polymer blend F8BT-TFB, which has aphotoluminescence efficiency of greater than 80%, photoluminescingyellow under white light, but also having an intrinsic yellowish colour,so that the material looks yellow even when the photoluminescence isquenched. This residual or intrinsic colour arises because the materialintrinsically absorbs a set of wavelengths which gives it a yellowappearance. This yellow colour is also apparent when the material isdeposited as a thin film as the material's absorption is still asignificant factor.

FIG. 2 a shows three spectra 200 illustrating the variation of lightintensity with wavelength for a material such as F8BT-TFB with anintrinsic colour. Spectrum 204 represents the photo-emission spectrum ofthe material in a device such as that shown in FIG. 1 a or 1 b with zeroapplied bias. With forward bias the spectrum shifts to spectrum 206,with an enhanced electroluminescent emission and a peak which is shiftedtowards longer (redder) wavelengths. When reverse bias is applied to adevice containing a material the spectrum shifts to spectrum 202,showing that the photoluminescent light emission is reduced in intensityand that the peak wavelength is shifted towards the blue.

FIG. 2 b, by contrast, shows a set of spectra 210 for a devicecontaining a photoluminescent material with no intrinsic colour.Spectrum 214 represents the device with no applied bias, spectrum 216the spectrum with forward bias applied with emission enhanced byelectroluminescence, and spectrum 212 the spectrum with reverse biasapplied to substantially quench the photoluminescence. As can be seenfrom FIG. 2 b the position of the peak of spectra 212, 214 and 216remains substantially constant as substantially the only contribution tothe device's colour arises from the emitted photo/electroluminescenceand not, as in FIG. 2 a, from any contribution from the photoluminescentlayer's intrinsic colour.

It will be appreciated that in FIGS. 2 a and 2 b the y-axis representsthe intensity of light emitted from a device such as shown in FIG. 1 aor 1 b, and therefore includes two components. A first component is thephoto or electroluminescent emission from layer 106 and a secondcomponent arises from reflection or scattering of the incidentillumination at layer 106. Thus in a device where, for example, thecathode is absorbing or transparent and relatively little light isscattered from the photoluminescent layer itself, the spectra of FIG. 2a may approach those of FIG. 2 b. Nonetheless it is preferable for someapplications to use a material which is intrinsically colourless ratherthan one which is intrinsically coloured as this simplifies devicedesign.

When selecting suitable photoluminescent materials the main criteria isa high photoluminesence efficiency, which assists in providing a displaywith good contrast. The skilled person will appreciate that althoughphotoluminescence efficiency and electroluminescence efficiency arerelated a high electroluminescence efficiency does not necessarily implya high photoluminescence efficiency. Important secondary considerationsin material selection are the material's inherent colour lifetimeprocessability, and stability. Significant decay routes inelectroluminescent materials include internal photo-degradation, excimerformation and electromigration. These are expected to be lesssignificant for photoluminescent materials in a reverse biased deviceand it is therefore practical to employ photoluminescent materials insuch a device which would be disregarded for use in anelectroluminescent device because of their poor lifetimes under forwardbias. Thus potentially a significantly broader range of materials isavailable to select from.

Red, green and blue intrinsically colourless photoluminescent polymers,as exemplified below, are available for use in photoluminescencequenching displays. This allows the production of colour displays aswell as black and white displays in which white illumination is producedby a blend of red, green and blue emitting photoluminescent polymers. Itwill be appreciated that since the photoluminescence quenching displayis an emissive display technology, albeit relying on quenching of anemission rather than electronic excitation of an emission, red, greenand blue emissive materials are desirable to provide a broad colourgamut For some applications, however, a sufficiently broad colour locusmay be provided using only two different colours of emission. The humaneye's visual sensitivity varies with wavelength and depends not only onthe peak position but also on, effectively, on a convolution of theintegrated area under the peak with the eye's sensitivity. This meansyellow emitters are generally perceived as brighter than, for example,blue emitters and this can also be taken into account when designing acolour display.

An example of an intrinsically colourless polymer with bluephotoluminescence is a polymer in the polyfluorene class, such as PFB orTFB.

An example of an intrinsically colourless red photoluminescent polymeris a polymer in the perylene class such as are disclosed in WO01/42331.An intrinsically red-brown polymer with red photoluminescence may beobtained by doping F8BT withpoly(2,7-(9,9-di-n-octylfluorene)-co-(2,5-thienylene-3,6-benzothiadiazole-2,5-thienylene).

An example of an intrinsically colourless green photoluminescent polymeris a polyquinoxaline such as those disclosed in WO01/55927.

An intrinsically colourless polymer with white photoluminescence may beprovided by blending the above intrinsically colourless polymers withred, green and blue photoluminescence in appropriate proportions.

FIGS. 3 a and 3 b show, schematically, an enlarged portion of across-section through a device such as that illustrated in FIG. 1 b; thephotoluminescent layer 106 in particular is not to scale. FIG. 3 a showsan embodiment comprising reflective a cathode layer 310 whilst in theembodiments of FIG. 3 b a transparent cathode layer 322 is used, behindwhich is optionally located an additional light absorbent layer 324. Inthe simplified diagrams of FIGS. 3 a and 3 b contacts to the anode andcathode are not shown, but arrangements such as described with referenceto FIG. 1 can be employed. Likewise cathodes 310 and 322 may compriseone or more layers as described with reference to FIG. 1 b, althoughwhere the cathode layers comprise metal the cathode layer 310 should bethick enough to be relatively light-reflective (typically >250 nm)whilst the cathode layer 322 should be thin enough to be relativelylight-transmissive in the visible region of the spectrum.

Referring now to FIG. 3 a in particular, incident white, blue or UVillumination 302 from either a front or back light or ambient lightpassed through the substrate 102, transparent anode 104, and holetransport layer 128 to the layer of photoluminescent material 106. Hereit is absorbed generating excitons, that is bound electron-hole pairssuch as schematically illustrated exciton 304. In other configurationsphotoluminescent layer 106 may be illuminated through the cathode ratherthan through the anode.

With no applied field a significant fraction of these optically excitedexcitons rapidly radiatively decay producing light according to thephotoluminescence spectra of the material or materials forming layer106. This photoluminescence, schematically illustrated at 108 in FIG. 3a, is emitted substantially isotropically, and thus the displayapproximates to a lambertian emitter. The fraction of the excitonsdecaying radiatively depends upon the photoluminescence efficiency ofthe material and upon the applied field. When the diode formed by thedevice is in an off state—typically, but not necessarily, when the anodeand the cathode are at the same electrical potential—the display is in aquiescent photo-emitting or on state.

In the arrangement of FIG. 3 a the cathode layer 310 reflects both aportion of the photoluminescence 308 and a portion of the incidentillumination 302. Another portion of incident illumination 302 isreflected or scattered directly by photoluminescent layer 106. Thus whenthe display is viewed an observer sees a combination of the emittedphotoluminescence 308 and a reflected and/or scattered component 306 ofthe incident illumination 302. This scattered light tends to reduce thedisplay contrast but can be substantially reduced by side illuminationas described later.

In FIG. 3 b cathode layer 322 is transparent and thus thereflected/scattered light component is much smaller or absent, althoughphotoluminescence emitted towards the rear of the device is effectivelylost. When the display is off (not photoluminescing) a viewer seesthrough the transparent cathode 322 to what is behind. Thus an absorbentor optically black layer 324 may be optionally provided behindtransparent cathode 322 or, in other embodiments, cathode 322 may itselfbe black. With the arrangement of FIG. 3 b back illumination ofphotoluminescent layer 106 is also possible but in this case, dependingupon the photoluminescence efficiency and upon the wavelength of theback-light, a viewer will generally see the back-light illuminationtogether with the photoluminescence, which again can reduce theeffective contrast.

When the diode formed by the anode, cathode and photoluminescent layer106 is reverse biased, that is when the anode is held at a lowerelectrical potential than the cathode, a fraction of the excitons aresplit into their constituent holes and electrons and these are thenconducted out of the structure with the aid of the applied electricfield. Thus this fraction of the excitons is prevented from radiativelydecaying and hence emitting photoluminescence. The fraction of theexcitons split apart in this way is determined by the reverse voltageapplied to the device, and thus the level of photoluminescence can becontrolled from a maximum value with no applied voltage to a reducedvalue depending upon the degree of reverse bias.

It will be appreciated that the power consumption of the device is verylow because, essentially, the only power required is that to conductaway the holes and electrons of the split excitons. This will varydepending upon the degree of incident illumination and upon thephotoluminescence efficiency. It will also be appreciated that since alarger reverse bias is needed for a dimmer display, the powerconsumption is to some degree dependent upon the degree of contrastrequired. It will further be appreciated that because the primary sourceof energy for the display is provided by the incident illumination, thedisplay will operate better in high ambient light conditions such asbright sunlight, conditions which conventional displays typically findhard to cope with. Where a material which is both a good photoluminescerand a good electroluminescer is selected a dual mode device is possiblein which, under bright conditions the display operates in aphotoluminescence quenching mode and in which under less brightconditions or conditions of no ambient illumination the device isforward biased to electroluminesce. One example of a material which hasboth a high photoluminescence efficiency and a high electroluminesenceefficiency is F8BT-TFB, and a dual mode device may be constructed usingthis material.

Referring now to FIG. 4, this shows exemplary illumination schemes forphotoluminescent quenching devices as described above. In FIG. 4 a adisplay device 400 comprises a photoluminescent quenching display 102,104, 106, 122, 128 with a back light 402. The back-light may comprise,for example, any conventional LCD back-light. The device anode layer 104comprises transparent indium tin oxide and cathode 122 a thin layer ofmetal, such as a 50 nm layer of calcium.

When back-light 402 is on and no reverse bias is applied to the displaydevice the display colour is a combination of the photoluminescencecolour and the intrinsic colour of photoluminesent layer 106. Thuswhere, for example; layer 106 is intrinsically colourless andphotoluminesces in blue, with a white back-light the display will appearblue-white with no bias and white with reverse bias. The same displayunder white ambient light illumination would appear blue with no biasand colourless (or would have the colour of the cathode) with reversebias.

It is possible to select the material of the photoluminescent layer 106in the device of FIG. 4 a so that the intensity of the colour changeswith the applied bias without a substantial colour shift. This can bedone by selecting a material in which the photoluminescence colourapproximates to the intrinsic colour of the material, that is by using amaterial with a set of spectra which is closer to that of FIG. 2 a thanthat of FIG. 2 b. One example of such a material is the yellow emitterF8BT-TFB. With such a configuration it is then possible to use theback-light 402 to compensate for a lack of ambient illumination withoutany significant change in the display appearance, increasing the backillumination as the ambient illumination from the front falls.

The arrangement of FIG. 4 b shows one way in which the display can beartificially illuminated from the front, using a baffle 416 andcylindrical lenses 412 and 414 to direct light from light sources (notshown) to either side of the display on to the front of the displaysurface. Preferably this illumination is arranged, as illustrated, toshine across the front of the display and into the display, rather thanout towards a viewer of the display. This arrangement is best suited tophotoluminescent materials with spectra as illustrated in FIG. 2 b, thatis to intrinsically colourless materials. The cathode layer 122 in thedevice of FIG. 4 b may be either reflective or transmissive as explainedwith reference to FIG. 3.

For many applications, the arrangement of FIG. 4 b is preferable to thatof FIG. 4 a since a front-lit device with a reflective cathode willgenerally make better use of the luminescence than a back-lit device.Furthermore, in the arrangement of FIG. 4 b, the photoluminescencematerial does not need to be colourless allowing selection from abroader range of materials which, as a category, will tend to have ahigher photoluminescence efficiency than colourless materials as theycan absorb a greater fraction of the available illumination.

FIG. 4 c shows an optical structure for 20 which may be formed on thefront face of substrate 102 to improve the absorption of ambient lightand to direct emitted light towards an observer of the display. In apreferred embodiment this optical structure comprises a microlens array,that is a regular arrangement of small lenses 422. Such an array can beformed by a range of conventional techniques including lithographyand/or replication on substrates such as silicon, glass, and plastic.Lens sizes typically range from 20 μm to around 1 mm in diameter withfocal ratios from f1 to f4. The UK National Physical Laboratorymanufactures such arrays to customer specifications. Other opticalstructures which may be used to enhance the display's appearance include“motheye” antireflection structures.

FIG. 5 illustrates one particularly advantageous illumination method, inwhich light is waveguided within the substrate 102. In this way theillumination can be substantially confined within the display device sothat substantially the only light emitted by the display arises fromphotoluminescence, thus increasing the potential contrast

FIG. 5 a shows a cross section through a photoluminescence quenchingdevice 500 with illumination means 501 positioned to one side of thedevice.

FIG. 5 b shows a plan view of a photoluminescence quenching device 500with illumination means 501 on two opposite sides of the device. Theillumination means 501 comprises a longitudinal illumination source 502and a cylindrical lens 504 which collects light from light source 502and directs this into the substrate 102 in such a way that the light iswaveguided within the substrate. Other conventional means of couplinglight into a waveguided mode of the substrate, such as a prism orgrating, may also be employed.

As can be seen in FIG. 5 a light propagating within the substrate iswaveguided by total internal reflection from a front surface 103 of thesubstrate and by reflection at reflective cathode 122. It will beappreciated that the illumination could also be waveguided by reflectionat other surfaces. For example, the front surface 103 of the substratecould be provided with a layer which reflects the illumination but notthe photoluminescence. Were the device structure to be inverted on thesubstrate, a reflective anode could be used instead of a reflectivecathode. Whatever arrangement is chosen for the illumination, however,energy must always be coupled from the illumination source 502 into thephotoluminescence layer 106.

Total internal reflection at substrate front surface 103 is achievedwhen the angle θ between a ray 506 and the normal to the surface is sinθ=n₂/n₁, where n₁ is the refractive index of the substrate and n₂ is therefractive index of air. For angles of incidence greater than θ thelight is totally internally reflected and hence substantially confinedwithin the device. The launch optics (in this case the cylindrical lens)are designed to ensure the illumination is directed only intowaveguiding modes of the substrate and is thereby not visible to theobserver.

FIG. 5 c shows an enlargement of the photoluminescence quenching device500 in which layers 122, 106, 128 and 104 are collectively shown aslayer 512, which is reflective at its back surface. FIG. 5 c alsoschematically illustrates a single pixel 510 of the photoluminescencequenching display 500. When no bias is applied to this pixelphotoluminescence is emitted from the pixel as shown (for the purposesof illustration photoluminescence from other parts of layer 106 isassumed to be quenched). The illuminating light is substantiallyconfined within the substrate so that all an observer sees is thephotoluminescence 514 from the pixel; when a reverse bias is applied tothe pixel this photoluminescence is switched off.

Where, as illustrated in FIG. 5 a, cathode 122 is reflective morephotoluminescence is directed out of the front surface of the displaywhen the pixel is on (because light emitted in the reverse direction isreflected towards the front surface), although a shiny cathode, or theintrinsic colour of the photoluminescent material will be visible whenthe pixel is off or reverse biased, (depending upon whether or not thephotoluminescent material is intrinsically coloured). Where a matt blackcathode or a transparent cathode and absorbing layer is employed in thedevice an observer will either see photoluminescence or a black pixel.An optional filter 508 may be provided in front of the display to filterout shorter wavelength components of any ambient light to reduce thecontribution of any intrinsic colour the photoluminescent material mayhave where the cathode is reflective.

In the arrangement of FIG. 5 non-illuminated edges of thephotoluminescent layer 106 may be made reflective to increase the numberof passes the light 506 makes through the substrate. Where absorption inthe photoluminescent material is strong and/or photoluminescentefficiency is higher or the display area large, light sources may beprovided on more than one side or on all sides of the display 500.Although it is theoretically possible to waveguide the illuminatinglight within only the active device layers (and not the substrate)strong absorption of the illumination makes this impractical.

FIG. 6 a shows an example of a pixellated display structure 600. Thisbroadly corresponds to the above described display structures exceptthat the photoluminescent layer 106 is pixellated, that is it is dividedinto a plurality of separate display elements 602. Likewise the cathodelayer or layers 122 is divided into a plurality of separate cathodes 604each with its own contact 606. The substrate 102, anode 104, and holetransport layer 128 are, however, common to all the pixels. Thus anindividual pixel may be switched off by applying a reverse bias betweenthe common anode 104 and the appropriate cathode connection 606. Inother pixelated displays X-Y pixel addressing may be employed using rowand column electrodes.

In the arrangement of FIG. 6 a the individual photoluminescent displayelements 602 may either have all the same colour, or may have differentcolours to provide a colour display. Alternatively a colour display maybe provided by using a white-emitting polymer blend for thephotoluminescent material for elements 602 and then appropriatelyfiltering the display elements to provide red, green, and blue pixels.Conventional filters such as are used for LCD displays can be employedfor this purpose, or additional filtering layers may be included withinthe structure of the pixellated device 600. Suitable filtering materialsare described in WO98/59529 and the materials mentioned therein whichfunction as filters are hereby incorporated by specific reference. As afurther alternative a combination of white and coloured photoluminescentpixels may be employed, the coloured pixels being used to directlyproduce a desired colour, and the white pixels being filtered to provideother colours. For example blue pixels may be provided using bluephotoluminescent material and red and green pixels may be provided byfiltering white photoluminescent emission.

Still further examples of device structures which may be driven inreverse to provide multicolour photoluminesent displays are provided inWO95/06400 (FIG. 1 and the accompanying description) and WO98/59529(FIG. 1 and the accompanying description, and the statement ofinvention) and these are included in the present application byreference to the specifically mentioned parts of these documents.

FIG. 6 b shows display equipment 610 including a colour pixellateddisplay 612, display driver circuitry 614 and a power sourceillustratively shown by battery 616. The display 612 comprises aplurality of red 618, green 620, and blue 622 pixels arranged in apattern which, from a distance, is capable of providing the appearanceof a variable colour display. A variety of pixel patterns are possiblein addition to the one shown to help reduce visual artefacts. Forexample a repeated pattern of four pixels, red, green, green and bluemay be employed.

Display driver 614 receives a display signal input 624 and provides anoutput 626 to drive electrodes 104 and 604 of FIG. 6 a. As illustratedin FIG. 6 b the common anode connection 104 and the negative terminal ofthe power source, battery 616, are both connected to ground. The displaydriver applies a positive voltage from power source 616 to a selectedcathode connection 606 in accordance with the display signal input online 624. The display signal may comprise a single pixel on/off signalor may comprise an analogue or digital pixel brightness signalindicating a desired level of pixel brightness between the on and offstates. In a colour display such as is illustrated in FIG. 6 b separatesignals are preferably provided for each red, green and blue pixel, togive the appearance of variable colour pixels.

When the display signal indicates that a pixel is to be on displaydriver 614 either leaves the appropriate pixel unbiased (zero bias), orapplies a forward bias, or applies a reverse bias to provide a degree ofphotoluminesence quenching to control the pixel to a predeterminedmaximum brightness. When the display signal indicates that a pixel is tobe off the display driver applies a reverse bias to the pixel topartially or completely quench the photoluminescence from the pixel, forexample to bring the pixel brightness down a predetermined offbrightness level. When the display signal indicates a desired pixelbrightness between maximum and minimum brightness the display circuitry614 applies an appropriate level of reverse bias to the selected pixelfor the desired pixel brightness.

The display driver 641 may also incorporate means to provide anadjustable duty cycle pulse-width modulated drive signal to each pixelresponsive to the display signal input on line 624. The pulse modulateddriving signal may have a zero or forward bias first voltage level and areverse bias second voltage level and a frequency of 60 Hz or greater.By selecting, for example, one of a plurality of mark-space ratiosprovided by a pulse generator the brightness level of a pixel may becontrolled and, in a colour display, the colour and luminescence orbrightness of a pixel may be controlled.

Referring now to FIG. 6 c, this shows an exemplary pulse-width modulated(PMW) waveform 630 for use in controlling pixel brightness. The waveformshows the voltage applied to a pixel against time, the voltage varyingbetween a first level 632, in the illustrated example zero volts, and asecond level 634, which in the illustrated example corresponds to fullreverse bias applied to the pixel. The portion of the waveform atvoltage level 632 is referred as the “mark” and the portion of thewaveform at level 634 as the “space”. During the mark portion of thewaveform, the pixel photoluminescences and during the space portion ofthe waveform, the photoluminescence is substantially quenched.

The frequency of waveform 630 is chosen so that rather than a pixelappearing to flash on and off, emission from the pixel appearssubstantially continuous, but with a brightness proportional to the onor mark period of the waveform. To achieve this, a frequency of at least25 Hz to 50 Hz is generally required. It can be seen from FIG. 6 c thatwhen the mark-to-space transition 636 is as shown, the pixel appears atapproximately 25% of its full brightness. Transition positions 638 and640 correspond, respectively, to pixel brightnesses of 50% and 75% and100% brightness corresponds to a steady state zero volts (in theexample) with a 100% mark:space ratio duty cycle. Waveforms other thanthat shown in FIG. 6 c may also be used and, for example, the drivingwaveform need not have square edges.

Using pulse width modulation has the advantage that there is asubstantially linear relationship between the duty cycle and theapparent-pixel brightness. Were the pixel brightness to be varied byvarying the reverse bias voltage the characteristics of individualpixels would need to be relatively closely matched and some form oflinearisation, such as a look-up table, might also be necessary. Anadditional or alternative form of brightness control comprisessub-dividing each pixel into n sub-pixels with area ratios in powers of2 (2⁰, 2¹, 2² etc), thus providing 2^(n) different brightness levelsdepending upon which sub-pixels are selected to be on.

In principle, every pixel in the display may have a different brightnessto the other pixels and thus the display driver 614 of FIG. 6 b shouldbe capable of driving each pixel with a pulse width modulated waveformappropriate for its selected brightness. One way of achieving this is toprovide a separate, variable pulse-width pulse generator for each pixelor for each row or column of pixels in the display. Suitable integratedcircuits to accomplish this are available from the Clare Micronixsubsidiary of Clare, Inc, California, USA and include the MXED101,MXED102, and MXED202. For example, the MXED102 is a 240 channelcascadable column driver providing 240 independently adjustable pulsewidth modulated outputs. Data sheets for these devices are available onthe Clare Micronix website and are hereby incorporated by reference.

The photoluminescence quenching effect switches on and off very quickly,which is generally advantageous, but which also makes the use of apassive matrix scanning-type display more difficult. In a passive matrixdisplay one of the electrodes is patterned in rows and the other incolumns and each pixel is addressed by applying an appropriate voltagebetween the row and column electrodes at whose intersection it lies. Inan LCD display the relatively slow response means that by the time thepixel is next activated, the state of the pixel is substantiallyunchanged. However, in a photoluminescence quenching display such ascanned arrangement results in a reverse bias only being applied for asmall fraction of the total time, thus reducing contrast. For thisreason, it is preferable to use an active matrix-type display. In anactive-matrix display, circuitry is provided so that each pixel can beleft in either an emitting or a non-emitting state whilst another pixelis addressed.

An exemplary active matrix pixel driver circuit 650 is shown in FIG. 6d. A photoluminescence quenching display pixel is schematicallyillustrated by a diode 652 connected between a zero volt bus 651 and aswitching transistor 656, in turn connected to a positive voltage supplybus 658. When switching transistor 656 is on, diode 652 is reversedbiassed by the voltage between busses 654 and 658. A storage capacitor660 stores charge to hold switching transistor 656 in a selected state,in a preferred embodiment either fully on or fully off. Charge is storedon capacitor 660 by means of a second transistor 662 connected to a row(or column) signal line 664 and to a column (or row) scan line 666. Whena voltage is applied to the scan line to switch transistor 662 on, thevoltage on signal line 664 is applied to storage capacitor 660, whichretains its charge state when transistor 662 is afterwards switched off.

The substrate 102 of the photoluminescence quenching device maycomprise, for example, either glass or plastic and the pixel drivercircuit 650 may be constructed using either amorphous silicon or organicconductors, capacitors, and transistors. The active matrix electronics,when integrated with the display pixels, may be located behind thereflective cathode or between the photoluminescence layer 106 and thesubstrate 102, in which case the photoluminescence quenching structurerather than the substrate is at the front (towards an observer) of thedisplay.

The photoluminescence quenching pixel draws virtually no current when itis reversed biased which facilitates the use of organic thin filmtransistors. Organic devices offer the additional advantages ofmaterials compatibility, ease of fabrication, flexibility and the likealso provided by the photoluminescence quenching display elements. Thefabrication of suitable devices is described in papers from the SID 2001Symposium in San Jose, Calif. of June 2001, ‘AMLCD Using OrganicThin-Film Transistors on Polyester Substrates’, M. G. Kane, I. G. Hill,J. Campi, M. S. Hammond, B. Greening (all of Sarnoff Corp), C. D.Sheraw, J. A. Nichols, D. J. Gundlach, J. R. Huang, C. C. Kuo, L. Jia,T. N. Jackson (Penn State Univ), J. L. West, J. Francl (Kent StateUniv), SID Symposium Digest, Vol. 32 pp 57-59, and in ‘All-Polymer ThinFilm Transistors Fabricated by High-Resolution Ink-Jet Printing’ T.Kawase, (Univ of Cambridge and Seiko-Epson Corp.), H. Sirringhaus, R. H.Friend (Univ. of Cambridge), T. Shimoda (Seiko-Epson Corp.), SIDSymposium Digest, Vol. 32, pp 40-43. Both these papers are herebyincorporated by reference.

Further details of pixel driving arrangements which may be adapted to aphotoluminescence quenching display are described in WO 99/42983assigned to the present applicant, and also in U.S. Pat. Nos. 5,828,429,5,903,246, and 5,684,365, all of which are hereby incorporated byreference.

Referring now to FIG. 7, this shows experimental apparatus 700 formeasuring the intensity of photoluminescence emitted by aphotoluminescence quenching display device as reverse bias is applied.

A xenon lamp 702 is coupled by a lens 704 to a monochromator 706, toallow the selection of a narrow range of illuminating wavelengths. Theoutput from monochromator 706 is then focussed via a pair of lenses 708,710 onto the display device-under-test 714. The lenses 708, 710 allowthe monochromator output to be modulated by a mechanical chopper wheel712 driven a lock-in amplifier 724. Photoluminescence fromdevice-under-test 714 excited by the illumination from monochromator 706is collected by lens 716 and directed onto a photodiode 720 also coupledto lock-in amplifier 724. The collected light is filtered by a low-passfilter 718 which rejects scattered light from monochromator 706 whilstallowing the photoluminescence to pass. A voltage source 722 is used toprovide a variable reverse bias voltage to device under test 714. Thelock-in amplifier 724 provides an output indicating the level ofphotoluminescence from device 714.

EXAMPLES

The results from two exemplary devices will be presented. The iscomprised an 80:20 polymer blend of F8BT:TFB with a two layercalcium/aluminium cathode. The second comprised a 79:20:1 polymer blendofF8BT:TFB:poly(2,7-(9,9-di-n-octylfluorene)-co-(2,5-thienylene-3,6-benzothiadiazole-2,5-thienylene)with a three layer cathode of lithium fluoride/calcium/aluminium. Bothdevices photoluminesced in the yellow and had an intrinsic yellowcolouration.

FIGS. 8 a and 8 b show the variation of photoluminescent emission withreverse bias for the first and second devices respectively. In each casethe devices were excited using light having a wavelength of 466 mm, frommonochromator 706, and filter 718 and photodiode 720 were arranged tocollect light of a wavelength longer than 570 nm. The two graphs havebeen normalised to a maximum 100% photoluminescence level at zeroapplied bias.

The two graphs show that with a reverse bias voltage of around 20 voltsthe photoluminescence is reduced to approximately half its initialvalue. The photoluminesence was observed to return to its originalintensity once the reverse bias voltage was removed.

FIG. 9 shows the variation of photoluminescence intensity as a functionof illuminating wavelength from monochromator 706 for the first device.The photoluminescence is cut off when the excitation wavelength islonger than around 570 nm; the residual tail on the graph of FIG. 9results from scattered light from the excitation source. It can be seenthat the maximum photoluminescence is observed when the excitationsource has a wavelength of between 400 nm and 500 nm. Thischaracterisation assists in selecting an appropriate illuminationsource. The threshold for photoluminescence in the device of FIG. 9, 570nm, corresponds to the minimum photon energy which can still generate anexciton in the photoluminescent material. Thus in devices where it isdesirable to prevent photoluminescence stimulated by ambient orbackground light a filter cutting off at wavelengths above 570 nm placedin front of the device will reduce ambient light-stimulatedphotoluminescence whilst still permitting photoluminescent emission atwavelengths longer than 570 nm to pass. This is helpful in devices ofthe type illustrated in FIG. 5 a.

FIG. 10 illustrates a theoretical mechanism which is believed to beresponsible for the photoluminescence quenching. Incident illuminationcauses a π-π* transition in one of the polymers, of the photoluminescentpolymer blend, F8BT, generating an exciton, that is a boundhole-electron pair. This exciton may be dissociated by thermal energygreater than the exciton binding energy E_(b). In an electric field theenergy required to dissociate an exciton is reduced to approximatelyE_(b)−Xed where X is the electric field, e the charge on an electron andd the distance by which the hole and electron must be separated in orderfor the dissociation to be complete.

Referring again to FIG. 10, this shows the vacuum energy level 1000 andthe lowest unoccupied molecular orbital (LUMO) energy levels 1002 and1004 for TFB and F8BT respectively. FIG. 10 also shows the highestoccupied molecular orbital (HOMO) energy levels 1006 and 1008 for TFBand F8BT respectively. In a simple picture, an exciton on F8BT willdissociate if the energy gained by a hole transferred to the HOMO of theTFB polymer (0.56 eV) exceeds the binding energy of the exciton on theF8BT polymer. Similarly an exciton formed on the TFB polymer willdissociate if the energy gained by transferring an electron to the LUMOof the F8BT polymer exceeds the binding energy of the exciton on the TFBpolymer. It is believed that by applying a reverse bias electric fieldthe energy needed to dissociate an exciton on the F8BT and on the TFBpolymer is reduced and thus this hole/electron transfer process isactivated—that is less energy is required for this transfer process andthus, at a given temperature, the process is more likely to occur.Dissociation must take place faster than radiative recombination.Measurements have determined that the estimated reduction in bindingenergy is consistent with the energy required to separate ahole-electron pair by a distance roughly equal to the separation betweenTFB and F8BT polymer chains.

No doubt many other effective alternatives would occur to the skilledperson and it will be understood that the invention is not limited tothe described embodiments and encompasses modifications apparent ofthose skilled in the art and within the spirit and scope of the claimsappended hereto.

1. A method of displaying information using photoluminescence quenching, the method comprising: providing an optoelectronic display comprising a photoluminescent material between a pair of electrodes; providing illumination for the photoluminescent material to cause the photoluminescent material to photoluminesce; and biassing the electrodes to at least partially quench said photoluminescence, the photoluminescent material having a normally-on, photoluminescence emissive state under zero bias across the electrodes.
 2. A method of displaying information as claimed in claim 1 wherein said photoluminescent material comprises an organic photoluminescent material.
 3. A method of displaying information as claimed in claim 2 wherein said organic photoluminescent material comprises a semiconducting conjugated organic polymer.
 4. A method as claimed in claim 3 wherein said pair of electrodes comprises a cathode and an anode, said anode having a higher work function than said cathode, wherein said photoluminescent material is sandwiched between said pair of electrodes and wherein said biassing comprises reverse biassing to make said anode more negative than said cathode.
 5. A method as claimed in claim 4 further comprising providing a hole transport material between said photoluminescent material and said anode.
 6. A method as claimed in claim 2 wherein said photoluminescent material becomes substantially colorless when said photoluminescence is quenched.
 7. A method as claimed in claim 2 wherein said photoluminescent material comprises a blend of materials having different photoluminescent colors.
 8. A method as claimed in claim 7 wherein said blend of materials photoluminesces with a substantially white color.
 9. A method as claimed in claim 1 wherein said optoelectronic display has a greater conductivity in a forward biassed direction of current flow between the electrodes than in a reverse biassed direction, and wherein said biassing comprises reverse biassing the electrodes.
 10. A method as claimed claim 1 wherein at least one of said electrodes is at least partially transparent, and wherein said method further comprising displaying said photoluminescence material through said at least partially transparent electrode.
 11. A method as claimed in claim 10 comprising providing said illumination through said at least partially transparent electrode.
 12. A method as claimed in claim 10 wherein the optoelectronic display includes a light absorbing material on an opposite side of said photoluminescent material to said at least partially transparent electrode, the method further comprising at least partially absorbing in said light absorbing material a portion of said illumination transmitted through said photoluminescent material.
 13. A method as claimed in claim 12 wherein said light absorbing material forms at least part of the other of said electrodes.
 14. A method as claimed in claim 10 wherein both said electrodes are at least partially transparent, the method further comprising using a backlight for providing said illumination.
 15. A method as claimed in claim 1 further comprising using ambient light for providing said illumination.
 16. A method as claimed in claim 1 wherein the photoluminescent material of said optoelectronic display extends in two dimensions further than in a third dimension measured between said pair of electrodes, to provide a display area, the method further comprising illuminating said photoluminescent material using light propagating generally perpendicular to said display area.
 17. A method as claimed in claim 1 wherein said optoelectronic display comprises a substrate mounting said photoluminescent material, the method further comprising illuminating said photoluminescent material by waveguiding light through said substrate.
 18. A method as claimed in claim 17 wherein said photoluminescent material is sandwiched between the two electrodes, one of said electrodes is transmissive to said illumination and said photoluminescence, the other of said electrodes is reflective at least to said illumination, said optoelectronic display is configured for viewing through a front surface of said substrate, said photoluminescent material is mounted on a rear surface of said substrate, said transmissive electrode is located closer to said front surface than said reflective electrode, and wherein said waveguiding comprises waveguiding in a waveguiding region between the front surface of said substrate and said reflective electrode.
 19. A method as claimed in claim 1, the method further comprising providing a plurality of photoluminescent display elements each having an associated pair of electrodes between which photoluminescent material is located, and biassing one or more of said pairs of electrodes to display information.
 20. A method as claimed in claim 19 wherein said display elements comprise photoluminescent material photoluminescing with different colors for displaying information using two or more colors.
 21. A method as claimed in claim 1 wherein said optoelectronic display comprises a pixellated display having pixels of at least two different colors positioned adjacent one another the pixels comprising at least two different respective photoluminescent materials photoluminescing at said different colors each pixel having associated electrodes, the method further comprising biassing the electrodes of said differently colored pixels to at least partially quench said differently colored photoluminescence to provide a multicolored, pixellated display.
 22. A method as claimed in claim 1 wherein said photoluminescent material is also electroluminescent, the method further comprising biassing said electrodes with an opposite polarity to said biassing to at least partially quench said photoluminescence, to cause said photoluminescent material to electroluminesce.
 23. A method as claimed in claim 1 further comprising providing said optoelectronic display with an optical structure to collect and deliver light to said photoluminescence material.
 24. A method as claimed in claim 23 wherein said optical structure comprises a plurality of micro-lenses.
 25. A method as claimed in claim 1, wherein said biassing comprises applying a voltage waveform to the electrodes, the method further comprising controlling a duty cycle of the waveform to adjust the quenching of said photoluminescence.
 26. Method of displaying information on an optoelectronic display, the method comprising providing an optoelectronic display comprising a photoluminescent material between a pair of electrodes the method, comprising illuminating the photoluminescent material to stimulate photoluminescence and; applying a voltage to the electrodes to quench the photoluminescence to display information, the photoluminescent material having a normally-on, photoluminescence emissive state under zero bias across the electrodes.
 27. Method as claimed in claim 26 wherein the display comprises a diode and the applied voltage reverse biasses the diode.
 28. Method as claimed in claim 26 in ambient light, comprising using the ambient light alone to stimulate photoluminescence in the display.
 29. Method as claimed in claim 26 wherein said optoelectronic display comprises dedicated illumination source to stimulate said photoluminescence.
 30. Method as claimed in claim 26, further comprising using pulse width modulation of the applied voltage to control said photoluminescence.
 31. Method of controlling a display driver of a light emitting display to display information, comprising: providing a light emitting display comprising a photoluminescent material between a pair of electrodes, applying a voltage of a first polarity from a display driver to the electrodes to reduce photoluminescence from the material to switch the light emission off and applying a reduced voltage at the first polarity, or substantially zero voltage, from the display driver to the electrodes to switch the light emission on, the photoluminescent material of the display having a normally-on, photoluminescence emissive state under zero bias across the electrodes.
 32. A method of operating a photoluminescent device, the device comprising a semiconductor layer in the form of a film of organic photoluminescent material, a first electrode layer proximate a first surface of the semiconductor layer, a second electrode layer proximate a second surface of the semiconductor layer, the method comprising illuminating the device and applying an electric field between the first and second electrode layers across the semiconductor layer so as to render the second electrode layer negative relative to the first electrode layer to split optically excited excitons generated by the illumination into their constituent holes and electrons and to conduct said holes and electrons out of said photoluminescent film to inhibit photoluminescence from said film, the photoluminescent material of the display having a normally-on, photoluminescence emissive state under zero bias across the electrode layers.
 33. A method as claimed in claim 32 wherein the film of organic photoluminescent material is coupled to said second electrode layer by a hole transport layer.
 34. A method as claimed in claim 32 wherein at least one of said first and second electrode layers is at least partially transparent, for viewing said photoluminescent material.
 35. A method as claimed in claim 34 wherein said device is illuminated through said at least partially transparent electrode layer.
 36. A method as claimed in claim 35 comprising illuminating the device using ambient light.
 37. A method as claimed in claim 34 comprising illuminating the device by guiding light within a region including said semiconductor layer.
 38. A method as claimed in claim 37 wherein said semiconductor layer and said first and second electrode layers are mounted on a substrate, and wherein said region is bounded by a front surface of said substrate and the one of the first and second electrode layers farthest from said front surface of said substrate.
 39. A method as claimed in claim 32 wherein said first electrode layer has a lower work function than said second electrode layer.
 40. A method as claimed in claim 32 wherein said organic photoluminescent material comprises at least one conjugated polymer.
 41. A method as claimed in claim 40 wherein said film of organic material comprises a thin, dense polymer film with a sufficiently low concentration of extrinsic charge carriers for the material to be suitable for an electroluminescent device.
 42. A method as claimed in claim 40 wherein said photoluminescence material is selected to be substantially colorless when said photoluminescence is substantially completely inhibited.
 43. An optoelectronic display device comprising: a semiconductor layer in the form of a film of organic photoluminescent material, a first electrode layer proximate a first surface of the semiconductor layer, a second electrode layer proximate a second surface of the semiconductor layer; and a light source to illuminate said photoluminescent material to stimulate photoluminescence from the material, the organic photoluminescent material of the display having a normally-on, photoluminescence emissive state under zero bias across the electrode layers.
 44. An optoelectronic display as claimed in claim 43 wherein the film of organic photoluminescent material is connected to said second electrode layer via a hole transport layer.
 45. An optoelectronic display as claimed in claim 43 wherein said organic photoluminescent material comprises at least one conjugated polymer.
 46. An optoelectronic display as claimed in claim 45 wherein said film of organic material comprises a thin, dens polymer film and wherein the polymer film of the semiconductor layer has a sufficiently low concentration of extrinsic charge carriers that on applying an electric field between the first and second electrode layers across the semiconductor layer so as to render the second electrode layer positive relative to the first electrode layer charge carriers are injected into the semiconductor layer and radiation is emitted from the semiconductor layer.
 47. An optoelectronic display as claimed in claim 45 wherein said photoluminescence material is selected to be substantially colorless when said photoluminescence is substantially completely inhibited.
 48. An optoelectronic display as claimed in claim 43 wherein at least one of said first and second electrode layers is at least partially transparent, for viewing said photoluminescent material.
 49. An optoelectronic display as claimed in claim 48 wherein said device is illuminated through said at least partially transparent electrode layer.
 50. An optoelectronic display comprising: a photoluminescent display device having a display on-state in which the display emits photoluminescence under optical illumination with no voltage applied to the device and a display off-state in which said photoluminescence is at least partially quenched; and device driver circuitry having an input for receiving a display signal and an output for driving said display device, said display signal having an on-state indicating that the display is to be on and an off-state indicating that the display is to be off; said photoluminescent display device comprising: a semiconductor layer in the form of a film of organic photoluminescent material, a first electrode layer proximate a first surface of the semiconductor layer, and a second electrode layer proximate a second surface of the semiconductor layer; and wherein said device driver circuitry is configured to apply an electric field between the first and second electrode layers across the semiconductor layer so as to render the second electrode layer negative relative to the first electrode layer to split optically excited excitons generated by the illumination into their constituent holes and electrons and to conduct said holes and electrons out of said photoluminescent film to inhibit photoluminescence from said film in response to said display signal having said off-state; said display device and device driver combination primarily operating to display information by photoluminescence quenching, the organic photoluminescent material of the display having a normally-on, photoluminescence emissive state under zero bias across the electrode layers.
 51. An optoelectronic display as claimed in claim 50 wherein said display photoluminescence is variable between said display on-state and said display off-state, wherein said display signal is variable between said display signal on-state and off-state, and wherein said device driver circuitry is configured to provide a variable waveform voltage output in accordance with said variable display signal to vary the average electric field between said first and second contact layers, to thereby vary said display photoluminescence.
 52. An optoelectronic display as claimed in claim 50 wherein said device driver circuitry is further configured to reduce but not to reverse said electric field in response to said display signal having said on-state.
 53. An optoelectronic display as claimed in claim 50 further comprising a light source to illuminate said photoluminescent material to stimulate photoluminescence from the material.
 54. An optoelectronic display as claimed in claim 53 wherein both said first and second electrode layers are at least partially transparent, and wherein said light source is situated behind said film of photoluminescent material when the display is viewed from the front.
 55. An optoelectronic display as claimed in claim 53 wherein said display device includes a region to channel illumination from said light source using internal reflection, to illuminate said photoluminescent material.
 56. Optoelectronic device driver circuitry as recited in claim
 50. 57. An optoelectronic display comprising: a semiconductor layer in the form of a film of organic photoluminescent material, a first electrode layer proximate a first surface of the semiconductor layer, and a second electrode layer proximate a second surface of the semiconductor layer; and a region to channel illumination from a light source using internal reflection, to illuminate said photoluminescent material, the organic photoluminescent material of the display having a normally-on, photoluminescence emissive state under zero bias across the electrode layers.
 58. An optoelectronic display as claimed in claim 57 wherein said semiconductor layer and said first and second electrode layers are mounted on a substrate, and wherein said illumination channelling region includes said substrate.
 59. An optoelectronic display as claimed in claim 58 wherein said illumination from said light source is substantially confined between a front surface of said substrate and the one of the first and electrode layers farthest from said front surface of said substrate.
 60. An optoelectronic display as claimed in claim 58 wherein said illumination is internally reflected by total internal reflection at an internal front surface of said substrate.
 61. An optoelectronic display comprising: a semiconductor layer in the form of a film of organic photoluminescent material, a first electrode layer proximate a first surface of the semiconductor layer, and a second electrode layer proximate a second surface of the semiconductor layer; a substrate carrying said semiconductor layer and said first and second electrode layers; and an optical structure on said substrate to collect and deliver light from said photoluminescent material to a viewer of the display, the organic photoluminescent material of the display having a normally-on, photoluminescence emissive state under zero bias across the electrode layers.
 62. An optoelectronic device as claimed in claim 61 wherein said optical structure comprises a plurality of micro-lenses.
 63. A pixellated optoelectronic display comprising a plurality of photoluminescent display devices each associated with a pixel of the display and having a pair of electrodes for addressing the device, and device driver circuitry for driving the electrodes to control the display, the pixels of the display having a normally-on, photoluminescence emissive state under zero bias across the electrodes, the display driver circuitry being configured to apply bias voltage to inhibit said photoluminescent emission from selected pixels of the display, to thereby display information.
 64. A pixellated optoelectronic display as claimed in claim 63 wherein said photoluminescent display devices comprise organic photoluminescent diodes, and wherein said bias voltage reverse biasses said diodes to inhibit said photoluminescent, emission.
 65. A pixellated optoelectronic display as claimed in claim 63 wherein said bias voltage is variable to variably inhibit said photoluminescent emission.
 66. A pixellated optoelectronic display as claimed in claim 63 comprising photoluminescent devices with a plurality of different colors whereby different pixels of the display are capable of displaying different colors.
 67. A pixellated optoelectronic display as claimed in claim 62, wherein said applied bias voltage has a variable duty cycle waveform and wherein the device driver circuitry has a display data input for controlling the relative brightness of said pixels, said device driver circuitry further comprising at least one variable duty cycle waveform generator responsive to said display data input to vary the duty cycle of the bias voltage waveform applied to a said pixel.
 68. An optoelectronic display operating on the principle of quenched photoluminescence, the display comprising: a first electrode; a second electrode; and a visible display element located between the first and second electrodes, the display element comprising photoluminescent material, the display being configured to at least partially quench photoluminescence from said photoluminescent material upon application of a voltage between said first and second electrodes and thereby visibly change from a photoluminescent emissive state to a reduced emissivity state to provide a visual display, the organic photoluminescent material of the display having a normally-on, photoluminescence emissive state under zero bias across the electrode layers.
 69. An optoelectronic display as claimed in claim 68 wherein said reduced emissivity state is a state in which, by comparison with said photoluminescent emissive state, visible photoluminescent emissivity from the photoluminescent material is substantially zero.
 70. An optoelectronic display as claimed in claim 68 wherein said photoluminescent material is substantially visually colorless in said reduced emissivity state.
 71. An optoelectronic display as claimed in claim 70 comprising a plurality of said visible display elements and a corresponding plurality of at least one of said first and second electrodes, a first subset of said visible display elements comprising a first photoluminescent material having a first color in said photoluminescent emissive state and a second subset of said visible display elements comprising a second photoluminescent material having a second color in said photoluminescent state, the visible display elements of said first subset being arranged near the visible display elements of said second subset, whereby the effect of a variable color display is created.
 72. An optoelectronic display in claim 71 wherein a third subset of said visible display elements comprises a third photoluminescent material having a third color, the visible display elements of said third subset being arranged near the visible display elements of said second subset.
 73. An optoelectronic display as claimed in claim 68 wherein one of said first and second electrodes is at least partially visually transparent.
 74. An optoelectronic display as claimed in claim 73 wherein both said first and second electrodes are at least partially visually transparent.
 75. An optoelectronic display as claimed in claim 68 wherein said first electrode, said second electrode and said photoluminescent material are selected such that the conductivity between the electrodes is greater when said first electrode is positive with respect to said second electrode, and wherein said second electrode is the positive electrode of said display and said first electrode is the negative electrode of the display.
 76. An optoelectronic display as claimed in claim 68 further comprising a first wire connected to said first electrode and a second wire is connected to said second electrode, wherein the display is photoluminescent when said photoluminescent material is illuminated and said photoluminescence is at least partially quenched when said first electrode is negative with respect to said second electrode, and wherein at least one of said first and second wires is marked to indicate that said second wire is to be positive with respect to said first wire to drive the display element off.
 77. An optoelectronic display as claimed in claim 68 further comprising instructions that said second wire is to be positive with respect to said first wire to drive the display element off.
 78. A combination of an optoelectronic display device and instructions, the display device comprising: a semiconductor layer in the form of a film of organic photoluminescent material, a first electrode layer proximate a first surface of the semiconductor layer, and a second electrode layer proximate a second surface of the semiconductor layer; the instructions comprising instructions to apply an electric field between the first and second contact layers across the semiconductor layer so as to render the second contact layer negative relative to the first contact layer to inhibit photoluminescence from said photoluminescent film, the organic photoluminescent material of the display having a normally-on, photoluminescence emissive state under zero bias across the electrode layers. 